1. Field of the Invention
The invention relates to methods and apparatus for digitizing a position of an instrument with respect to a grid of spaced conductors, and more particularly to low noise methods and apparatus for sequentially sensing induced signals in grid conductors of a digitizer.
2. Description of the Prior Art
Various high resolution apparatus for translating a position of a movable instrument, such as a pointer or pen, into electrical signals for transmission to a local or remote utilization device are well known in the art. Corresponding movements of the pointer or pen thus can be re-created at another location. Such apparatus, commonly referred to as digitizers, can also be utilized as data entry systems for computers. The known digitizing devices generally include a rigid support structure suitable for supporting a work sheet such as a graph, chart or piece of industrial artwork to be digitized. A grid or matrix of conductors is ordinarily imbedded immediately beneath the support surface. The moveable instrument is generally positioned over and pressed against the work sheet and is moved to trace various patterns or identify various coordinates on the work sheet. One known type of digitizer energizes the imbedded conductors, which are arranged as perpendicular groups of X and Y conductors forming a rectangular coordinate system, by means of a phase locked loop system. A voltage controlled oscillator drives a circuit which produces a predetermined phase change uniformly distributed between a group of parallel conductors in the grid or matrix by means of a precision resistive divider network. The pointer or pen operates as an antenna which electrostatically senses the composite electrical field at the location of the pointer or pen tip and feeds sense signals back to a phase detector of the phase locked loop system. The phase detector causes the oscillator frequency to be increased to seek a zero phase shift condition. The oscillator frequency therefore represents the location of the pointer or pen tip with respect to the group of spaced parallel conductors. The state of the art for this type of digitizer is indicated by U.S. Pat. Nos. 3,767,858; 3,851,097; 3,983,322; 3,886,311; and 4,022,971, all assigned to the present assignee. Although precision digitizers have been successfully implemented and marketed using the techniques described in the above patents, such digitizers are very complex and expensive, and further require a considerable amount of adjustment and maintenance. They are therefore unsuitable for many applications wherein a much less expensive and complex digitizer would be highly desirable. The above described type of digitizer further has the shortcoming that its accuracy is adversely affected by conductive marks or areas on the worksheets utilized.
Another known type of digitizer utilizes a coil located around the pointer or pen tip. The grid conductors imbedded in the support surface are sequentially energized with signals of a fixed predetermined amplitude and frequency for one of the X and Y groups of conductors. The voltage induced in the coil is inputted to circuitry which detects a polarity reversal of the signal induced in the coil as conductors on opposite sides of the pen are sequentially energized, in accordance with electromagnetic theory. The phase reversal detecting circuitry produces a pulse which is utilized to sample a position counter which counts at a predetermined rate during the scanning procedure. The sampled contents of the position counter represent the position of the pointer or pen tip for the X or Y coordinate being scanned. The state of the art for this type of digitizer is set forth in U.S. Pat. Nos. 3,735,044; 3,904,822; and 4,054,746. Although this type of digitizing system does not require the precision resistor arrays of the previously described digitizing system, it has a relatively low signal to noise ratio, and requires expensive high gain precision amplifier and phase shift circuitry for detecting the very low magnitude low energy signal induced in the pen tip coil. The foregoing type of digitizing system requires power transistors coupled to each grid conductor in order to drive a sufficiently large current pulse down each grid conductor to induce a suitably large signal in the coil supported by the pen tip. This results in an undue amount of noise in the form of switching transient voltages and also results in an undue amount of spurious electromagnetic radiation. Further, expensive and complex amplifying and shaping circuitry is required to process the signal induced in the pen tip coil.
Thus, there is presently an unmet need for an accurate but very low cost digitizer which requires minimal adjustment and maintenance for utilization in low cost applications such as replacement for light pens, trackballs, and joy sticks for CRT (cathode ray tube) cursor control, and which has a high signal-to-noise ratio.
Accordingly, it is an object of the invention to provide a low noise digitizing system which is substantially less expensive and more reliable than digitizing systems of the prior art.
It is another object of the invention to provide a digitizer having substantially higher signal-to-noise ratios than digitizers of the prior art.
It is yet another object of the invention to provide a digitizing system and method to produce a low noise sensing signal which can be conveniently amplified and analyzed to determine when grid conductors on opposite sides of the pen are sequentially scanned.
It is another object of the invention to reduce the amount of multiplexing circuitry required for a digitizer of the type having signals induced in a grid of conductors by a coil positioned above the grid.